Molecular cloning and partial characterization of the coxsackievirus B3 genome. Brief report

Mutations in the 5' NTR and the Non-Structural Protein 3A of the Coxsackievirus B3 Selectively Attenuate Myocarditogenicity

The 5’ non-translated region (NTR) is an important molecular determinant that controls replication and virulence of coxsackievirus B (CVB)3. Previous studies have reported many nucleotide (nt) sequence differences in the Nancy strain of the virus, including changes in the 5’ NTR with varying degrees of disease severity. In our studies of CVB3-induced myocarditis, we sought to generate an infectious clone of the virus for routine in vivo experimentation. By determining the viral nt sequence, we identified three new nt substitutions in the clone that differed from the parental virus strain: C97U in the 5’ NTR; a silent mutation, A4327G, in non-structural protein 2C; and C5088U (resulting in P1449L amino acid change) in non-structural protein 3A of the virus leading us to evaluate the role of these changes in the virulence properties of the virus. We noted that the disease-inducing ability of the infectious clone-derived virus in three mouse strains was restricted to pancreatitis alone, and the incidence and severity of myocarditis were significantly reduced. We then reversed the mutations by creating three new clones, representing 1) U97C; 2) G4327A and U5088C; and 3) their combination together in the third clone. The viral titers obtained from all the clones were comparable, but the virions derived from the third clone induced myocarditis comparable to that induced by wild type virus; however, the pancreatitis-inducing ability remained unaltered, suggesting that the mutations described above selectively influence myocarditogenicity. Because the accumulation of mutations during passages is a continuous process in RNA viruses, it is possible that CVB3 viruses containing such altered nts may evolve naturally, thus favoring their survival in the environment.

Toward testing the hypothesis that group B coxsackieviruses (CVB) trigger insulin-dependent diabetes: inoculating nonobese diabetic mice with CVB markedly lowers diabetes incidence

Insulin-dependent (type 1) diabetes mellitus (T1D) onset is mediated by individual human genetics as well as undefined environmental influences such as viral infections. The group B coxsackieviruses (CVB) are commonly named as putative T1D-inducing agents. We studied CVB replication in nonobese diabetic (NOD) mice to assess how infection by diverse CVB strains affected T1D incidence in a model of human T1D. Inoculation of 4- or 8-week-old NOD mice with any of nine different CVB strains significantly reduced the incidence of T1D by 2- to 10-fold over a 10-month period relative to T1D incidences in mock-infected control mice. Greater protection was conferred by more-pathogenic CVB strains relative to less-virulent or avirulent strains. Two CVB3 strains were employed to further explore the relationship of CVB virulence phenotypes to T1D onset and incidence: a pathogenic strain (CVB3/M) and a nonvirulent strain (CVB3/GA). CVB3/M replicated to four- to fivefold-higher titers than CVB3/GA in the pancreas and induced widespread pancreatitis, whereas CVB3/GA induced no pancreatitis. Apoptotic nuclei were detected by TUNEL (terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling) assay in CVB3/M-infected pancreata but not in CVB3/GA-infected pancreata. In situ hybridization detected CVB3 RNA in acinar tissue but not in pancreatic islets. Although islets demonstrated inflammatory infiltrates in CVB3-protected mice, insulin remained detectable by immunohistochemistry in these islets but not in those from diabetic mice. Enzyme-linked immunosorbent assay-based examination of murine sera for immunoglobulin G1 (IgG1) and IgG2a immunoreactivity against diabetic autoantigens insulin and HSP60 revealed no statistically significant relationship between CVB3-protected mice or diabetic mice and specific autoimmunity. However, when pooled sera from CVB3/M-protected mice were used to probe a Western blot of pancreatic proteins, numerous proteins were detected, whereas only one band was detected by sera from CVB3/GA-protected mice. No proteins were detected by sera from diabetic or normal mice. Cumulatively, these data do not support the hypothesis that CVB are causative agents of T1D. To the contrary, CVB infections provide significant protection from T1D onset in NOD mice. Possible mechanisms by which this virus-induced protection may occur are discussed.

A group B coxsackievirus/poliovirus 5' nontranslated region chimera can act as an attenuated vaccine strain in mice

The linear, single-stranded enterovirus RNA genome is flanked at either end with a nontranslated region (NTR). By replacing the entire 5' NTR of coxsackievirus B3 (CVB3) with that from type 1 poliovirus, a progeny virus was obtained following transfection of HeLa cells. The chimeric virus, CPV/49, replicates like the parental CVB3 strain in HeLa cells but is attenuated for replication and yield in primary human coronary artery endothelial cell cultures, in a human pancreas tumor cell line, and in primary murine heart fibroblast cultures. Western blotting analyses of CPV/49 replication in murine heart fibroblast cultures demonstrate that synthesis of CPV/49 proteins is significantly slower than that of the parental CVB3 strain. CPV/49 replicates in murine hearts and pancreata, causing no disease in hearts and a minor pancreatic inflammation in some mice that resolves by 28 days postinoculation. A single inoculation with CPV/49 induces protective anti-CVB3 neutralizing antibody titers that completely protect mice from both heart and pancreatic disease when mice are challenged 28 days p.i. with genetically diverse virulent strains of CVB3. That a chimeric CVB3 strain, created from sequences of two virulent viruses, is sufficiently attenuated to act as an avirulent, protective vaccine strain in mice suggests that chimeric genome technology merits further evaluation for the development of new nonpoliovirus enteroviral vectors.

Coxsackievirus B3 clinical isolates and murine myocarditis

Fifteen clinical coxsackievirus B3 (CVB3) isolates were assessed for cardiopathologic capabilities in adolescent male CD-1 mice in comparison to two well characterized cardiovirulent CVB3 strains. One isolate was cardiovirulent, one minimally cardiovirulent and the remaining 13 isolates were noncardiovirulent. The two cardiovirulent isolates and one well characterized cardiovirulent strain, established higher viremic titers, in comparison to five noncardiovirulent isolates that were examined. The two cardiovirulent isolates and one well characterized cardiovirulent strain replicated to significantly higher titers than five noncardiovirulent isolates in primary cultures of murine neonatal or adolescent cardiac fibroblasts. Nucleotide sequence analysis of an area defined by nucleotides(N)300-N599 in the 5'-nontranslated region were performed on the two well characterized cardiovirulent CVB3 strains, the two cardiovirulent isolates and 12 noncardiovirulent isolates. The data detected a single discriminatory nucleotide position. An A was present at N565 in three of four cardiovirulent CVB3, whereas a U or C was present in this position in 12 of 12 noncardiovirulent CVB3. In toto, these data are compatible with the hypothesis that the type of the nucleotide at N565, a position within the internal ribosome entry site, is associated with capacity of a CVB3 for replication in vivo and in vitro and this capacity for vigorous replication is associated with cardiovirulence.

The cardiovirulent phenotype of coxsackievirus B3 is determined at a single site in the genomic 5' nontranslated region

We report the construction of chimeric coxsackievirus B3 (CVB3) strains in which sequences of an infectious cDNA copy of a noncardiovirulent CVB3 genome were replaced by the homologous sequences from a cardiovirulent CVB3 genome to identify which of 10 predicted genetic sites determine cardiovirulence. Cardiovirulent phenotype expression was consistently linked to nucleotide 234 (U in cardiovirulent CVB3 and C in avirulent CVB3) in the 5' nontranslated region. Reconstructions of the parental noncardiovirulent CVB3 genome from chimeras restored the noncardiovirulent phenotype when tested in mice. Inoculation of severe combined immunodeficient (scid) mice with the noncardiovirulent CVB3 strain resulted in massive cardiomyocyte necrosis in all animals. Sequence analysis of viral genomes isolated from twelve scid mouse hearts showed that only nucleotide position 234 was different (a C-->U transition) from that in the input parental noncardiovirulent CVB3 genome. Higher-order RNA structures predicted by two different algorithms did not demonstrate an obvious local effect caused by the C-->U change at nucleotide 234. Initial studies of parental and chimeric CVB3 replication in primary cultures of fetal murine heart fibroblasts and in adult murine cardiac myocytes demonstrated that viral RNA transcriptional efficiency is approximately 10-fold lower for noncardiovirulent CVB3 than for cardiovirulent CVB3. CVB3 did not shut off protein synthesis in murine cardiac fibroblasts, nor were levels of viral protein synthesis significantly different as a function of viral phenotype. Taken together, these data support a significant role for determination of the CVB3 cardiovirulence phenotype by nucleotide 234 in the 5' nontranslated region, possibly via a transcriptional mechanism.

Coxsackievirus B3 from an infectious cDNA copy of the genome is cardiovirulent in mice

A coxsackievirus B3 (CVB3) cDNA clone, upon transfection of HeLa cells, produces CVB3 capable of induction of cardiac inflammation in C3H/He mice by day 8 post inoculation (p.i.). Liver and serum are cleared of CVB3 by day 8 p.i., but CVB3 persists in the heart through day 14. The nucleotide sequence and the predicted amino acid sequence of this clone have sites of divergence from 2 other completely sequenced CVB3 genomes although overall identity of the three CVB3 genomes is 99%.

Cell-free expression of the coxsackievirus 3C protease using the translational initiation signal of an insect virus RNA and its characterization

We have expressed the 3C protease of coxsackievirus B3 (CVB3) in a cell-free system. This expression system employs the translational initiation signal of an insect virus RNA, black beetle virus (BBV) RNA 1, to direct CVB3-specific protein synthesis. Using this expression system, we demonstrate that a biologically active 3C protease is synthesized which possesses both cis and trans processing capabilities. This in vitro-synthesized 3C protease is analogous to the native 3C, which was obtained from cytoplasmic extracts of CVB3-infected HeLa cells, in all biological parameters that were evaluated. In addition, antibody prepared against the 3C protease purified from extracts of CVB3-infected HeLa cells cross-reacts with the 3C protease produced in this cell-free system. Using the translational initiation signal from BBV RNA 1, we also have expressed the CVB3 capsid precursor and part of the P2 region in vitro, and have shown that the capsid precursor is cleaved between 1C (VP3) and 1D (VP1) by the proteolytic activity of in vitro-synthesized 3C in trans. Evidence also is presented to implicate the 2A protein of CVB3 as having proteolytic function.

Expression of functional beta-galactosidase containing the coxsackievirus 3C protease as an internal fusion

Alpha complementation of beta-galactosidase (beta gal) is intracistronic and requires interaction between the alpha donor region (residues 3-41) and alpha acceptor fragment (produced by M15). We have constructed two plasmids which direct the synthesis of hybrid beta gal: coxsackievirus proteins in Escherichia coli. One plasmid, pBD1045, encodes an enzymatically active 3C protease of coxsackievirus B3 fused between the amino-terminal 79 amino acids of beta gal (containing the alpha donor region) and amino acids 80 to 1023 (alpha acceptor region). A second plasmid, pBD1043 encodes an inactive 3C protease and results in a fusion of 260 coxsackievirus amino acids between residues 79 and 80 of the beta gal monomer. Both hybrid proteins expressed by these constructs have beta-galactosidase activity regardless of whether the viral protease (183 amino acids) is autocatalytically cleaved out of the chimeric protein (pBD1045) or remains as part of a fusion protein (pBD1043). The implications of these results for structural flexibility of the complemented beta-galactosidase enzyme are discussed.

Enteroviral infection in end stage dilated cardiomyopathy

In order to evaluate the role of enteroviral infections in end stage dilated cardiomyopathy, RNA was isolated from left ventricular myocardium of patients with dilated cardiomyopathy explanted during heart transplantation (n = 6) and from control hearts (n = 8), then probed using a dot blot procedure with two well-defined enteroviral cloned cDNA probes. One of the cardiomyopathic heart samples hybridized with the enteroviral probes, while RNA samples from the other diseased heart and the control heart demonstrated no hybridization. To verify further the enteroviral infection, a cDNA prepared from the positive heart RNA hybridized with Southern blotted coxsackievirus B3 and poliovirus 1 nucleotide sequences, while a control sample, which gave negative results in the dot blot, showed no hybridization to the enteroviral sequence. These results provide evidence for an enteroviral infection in certain patients with end stage dilated cardiomyopathy.

Echovirus 22 is an atypical enterovirus

Although echovirus 22 (EV22) is classified as an enterovirus in the family Picornaviridae, it is atypical of the enterovirus paradigm, typified by the polioviruses and the coxsackie B viruses. cDNA reverse transcribed from coxsackievirus B3 (CVB3) RNA does not hybridize to genomic RNA of EV22, and conversely, cDNA made to EV22 does not hybridize to CVB3 genomic RNA or to molecular clones of CVB3 or poliovirus type 1. EV22 cDNA does not hybridize to viral RNA of encephalomyocarditis virus or to a molecular clone of Theiler's murine encephalomyelitis virus, members of the cardiovirus genus. The genomic RNA of EV22 cannot be detected by the polymerase chain reaction using generic enteroviral primers. EV22 does not shut off host cell protein synthesis, and the RNA of EV22 is efficiently translated in vitro in rabbit reticulocyte lysates. Murine enterovirus-immune T cells recognize and proliferate against EV22 as an antigen in vitro, demonstrating that EV22 shares an epitope(s) common to enteroviruses but not found among other picornaviruses.

Murine cell-mediated immune response recognizes an enterovirus group-specific antigen(s)

Splenocytes taken from mice inoculated with coxsackievirus B3 (CVB3) (Nancy) developed an in vitro proliferative response against CVB3 antigen. This response could not be detected earlier than 8 days postinoculation but could be detected up to 28 days after exposure to CB3. CVB3-sensitized splenocytes responded not only to the CVB3 antigen but to other enteroviruses as well. This response was found to be enterovirus specific in that no response was detected to a non-enteroviral picornavirus, encephalomyocarditis virus, or to an unrelated influenza virus. The generation of a splenocyte population capable of responding to an enterovirus group antigen(s) was not limited to inoculation of mice with CVB3, as similar responses were generated when mice were inoculated with CVB2. Cell subset depletions revealed that the major cell type responding to the enterovirus group antigen(s) was the CD4+ T cell. Current evidence suggests that the group antigen(s) resides in the structural proteins of the virus, since spleen cells from mice inoculated with a UV-inactivated, highly purified preparation of CVB3 virions also responded in vitro against enteroviral antigens.

Chimeric picornavirus polyproteins demonstrate a common 3C proteinase substrate specificity

Cross-species proteolytic processing was demonstrated by the 3C proteinases of human rhinovirus 14 and coxsackievirus B3 on poliovirus-specific polypeptide precursors. Chimeric picornavirus cDNA genomes were constructed in a T7 transcription vector in which the poliovirus 3C coding region was substituted with the corresponding allele from human rhinovirus 14 or coxsackievirus B3. In vitro translation and processing of the polypeptides encoded by the chimeric genomes demonstrated that the proteolytic processing of poliovirus P2 region (nonstructural) proteins could be functionally substituted by the heterologous proteinases. In contrast, the 3C proteinase activities expressed from the chimeric genomes were incapable of recognizing the poliovirus-specific processing sites within the capsid precursor. Since the amino acid sequences flanking and inclusive of the P2 region cleavage sites of the three viruses are not stringently conserved, these results provide evidence for the existence of common conformational determinants necessary for 3C-mediated processing.

Defined recombinants of poliovirus and coxsackievirus: sequence-specific deletions and functional substitutions in the 5'-noncoding regions of viral RNAs

We describe the isolation of a variant of a polio--coxsackie recombinant virus (PCV110) containing a genomic RNA with a chimeric 5'-noncoding region. The variant virus [designated PCV110(1)] has growth and biosynthetic properties that are quite different from the original, temperature-sensitive isolate of the recombinant virus [designated PCV110(4)]. Nucleotide sequencing of the 5'-noncoding region of RNA from PCV110(1) revealed a 4-base deletion within the substituted coxsackievirus region of the chimeric genome that may contribute to the loss of temperature sensitivity of this variant recombinant virus. In addition, we have generated new recombinant viruses that contain (1) coxsackievirus sequences within the N66-N627 region of the poliovirus genome and (2) coxsackievirus sequences substituted from N1-N627 in the poliovirus genome. These recombinant viruses are not temperature sensitive for growth at 37 degrees and have biosynthetic properties similar to those of wild-type poliovirus. Our results provide evidence that replicase recognition signals encoded in the 5' noncoding regions of enterovirus genomic RNAs are not strictly sequence specific.

Rapid identification of coxsackie B viruses after immunoprecipitation and nucleic acid hybridization

To circumvent the difficulties in concentrating sufficient virus from a clinical or environmental sample for detection and identification, we have used immunoprecipitation to rapidly concentrate coxsackie B viruses from both large and small sample volumes. Antiviral serum and killed Staphylococcus aureus cells as a protein A source were used to bind and collect the virus. Radioactively-labeled viral nucleotide sequences were used to identify the collected virus by nucleic acid hybridization. The technique is applicable to the rapid concentration of dilute viruses from extremely large sample volumes as well as the samples from which virus isolation can be difficult. the process requires less than 2 days to complete and should be adaptable to virus identification by cell culture or other standard means as well.

A chimeric plasmid from cDNA clones of poliovirus and coxsackievirus produces a recombinant virus that is temperature-sensitive

We have inserted a 405-nucleotide fragment from the 5' noncoding region of the coxsackievirus B3 genome into an infectious cDNA copy of the poliovirus RNA genome. Transfection of plasmid DNA containing this hybrid genome construct into cultured monkey cells produced infectious virus. Recombinant virus stocks displayed a temperature-sensitive phenotype for growth at 37 degrees C. We found that there is a dramatic reduction in the level of viral proteins and viral RNAs in HeLa cells infected with the recombinant at 37 degrees C compared to that obtained at 33.5 degrees C. Thus, insertion of a portion of the coxsackievirus genome into the poliovirus genome produces a temperature-sensitive recombinant virus. That this substitution occurs in a region of the poliovirus genome that, to date, has not been shown to have any coding function suggests that RNA sequences involved in replicase recognition or ribosome binding may contribute to the temperature-sensitive phenotype of the recombinant virus.

Coxsackievirus B3: primary structure of the 5' non-coding and capsid protein-coding regions of the genome

The nucleotide sequence from the 5' terminus to nucleotide 3822 has been determined for the genome of the enterovirus, coxsackievirus B3 (CB3). This region encompasses the 5'-terminal 738 nucleotide non-coding sequence and the region which codes for the four viral capsid proteins and for the initial products of the P2 region. Regions in the 5' non-translated RNA sequence which may be involved with a structural and/or regulatory role are discussed.